Immune responses to microorganisms and rejection of allografts occur as a result of the activation of antigen specific lymphocytes. Activation of T lymphocytes is regulated by intermolecular binding events that occur at the interface of antigen-specific T cell and an antigen presenting cell. Although the T cell antigen receptor (TCR) plays a major role in regulating antigen-specific T cell activation, other cell surface molecules contribute to this activation process. One particular molecule of interest which this proposal focuses on is CD28, a homodimer of 44kD monomers. The anti-CD28 monoclonal antibody 9.3, at high concentrations can activate T cells. However, it is more potent in synergizing with agonists that bind to the TCR or with pharmacologic agents which mimic TCR-mediated signal transduction. Although TCR-mediated activation of T cells is cyclosporin A sensitive, activation via CD28 is not. These results suggest that CD28 regulates a signal transduction event distinct from that of the TCR. Thus, CD28 may regulate an important costimulatory pathway which contributes to T cell activation. This application proposes to examine the function of CD28 in T cell activation. Specific aims will focus on the role of CD28 on the transcriptional regulation of interleukin 2 (IL-2) and other lymphokine genes, on the role of CD28 in physiologic antigen responses, on structural features of CD28 that are important in its function and on identifying the signal transduction events mediated by CD28. Preliminary results demonstrate that CD28 can increase the activity of the IL-2 enhancer. We have identified a CD28-induced nuclear factor that binds to the IL-2 enhancer. We propose to identify the site of enhancer that is regulated by CD28 and characterize the factor that binds to it. We will determine whether this mechanism of transcriptional regulation is important for other lymphokines. Moreover, we propose to use this transcriptional regulatory mechanism as a probe to examine the importance of CD28 in antigen specific responses. Using a cell reconstitution system which we characterize in our preliminary results, we propose to identify the regions of the CD28 molecule that are important in signal transduction. These studies will be extended to characterize the signal transduction pathway that is regulated by CD28.